DMD2015-8729 Proceedings of the 2015 Design of Medical Devices Conference of the University of Minnesota DMD2015

April 13-16, 2015, Minneapolis, Minnesota, USA

 

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Principles of Within Electrode Current Steering (WECS) Niranjan Khadka1, Dennis Q. Truong1, Marom Bikson1 Department of Biomedical Engineering, The City College of New York, CUNY1

1 Background Transcranial Direct Current Stimulation (tDCS) is a neuromodulation technique that involves non-invasive delivery of weak direct current (1-2 mA) to the brain. Conventionally, tDCS employs rectangular saline-soaked sponge pads (25-35 cm2) placed on the scalp, with an internal electrode connected to the current source. Impedance measurement across the current source output may fail to recognize non-uniform conditions at the skin interface such an uneven content or saturation. tDCS is well tolerated with minor adverse effects limited to transient skin irritation [1]. Nonetheless, technology that enhances the sophistication of electrode design would further enhance tolerability and promote broad (e.g. home) use. In order to enhance the reliability and tolerability of tDCS, we describe a novel method called Within Electrode Current Steering (WECS). This concept is distinct from (across electrode) current steering, as developed for implanted devices such as Deep Brain Stimulation (DBS), where current is steered between electrodes that are each in contact with tissue, with the goal of changing desired brain regions that are activated [2]. WECS adjusts current between electrodes not in contact with tissue but rather embedded in an electrolyte on the body surface. The goal here is not to alter brain current flow, but rather compensate for non-ideal conditions at the surface. This technology leverages our technique for independently isolating electrode impedance and over-potential during multi-channel stimulation [3]. With a novel approach, the objective of this first paper was to demonstrate the principles of WECS using an exemplary electrode design typical for tDCS (4 rivet-electrode sponge) and extremes of current steering (from uniform in all rivets to a single rivet). Through Finite Element Method (FEM) simulation of this illustrative case, we validate the underlying assumptions of WECS: steering current within electrodes but without altering current distribution in brain target. Having presented this novel idea through an exemplary case, this report supports future studies in optimization of electrode design, automation of algorithms to control current (including using impedance measurement), and ultimately validation under experimental conditions.

2 Methods PRINCIPLES: WECS applies to non-invasive electrical stimulation with two or more electrodes (metal-rivets) embedded in an electrolyte (saline or gel)) on the skin [4]. Each electrode is independently powered by a current source. Success in implementation of WECS depends on geometry and material of each component of the assembly and an

algorithm for current steering between electrodes. Here our goal is only to demonstrate the principle of such application through a case design. Using a multi-scale model including a realistic electrode and head geometry (Fig 1A), we showed how current flow in the brain (target) is independent of current steering at the electrode. ELECTRODE DESIGN: To illustrate implementation of WECS, we use a modified tDCS saline-saturated sponge (7x5x3cm, σ = 1.4S/m). The top face of the sponge is perforated with cylindrical Ag/AgCl electrodes (dout= 1.5cm, din= 0.61cm, extrusionouter =1cm, and extrusioninner = 0.50cm σ = 5.99E7S/m), which align with the top surface and protrude through half the sponge thickness (Fig 1A). The electrodes are exposed on all surfaces and connect the lead (not shown) via male receptacles at the top. In principle, changing the diameter and distance between the electrodes, the distance between the electrodes and skin, or electrolyte conductivity will discriminate how current from the electrode reaches the skin [5], but here our goal is to illustrate WECS principles in one fixed exemplary geometry. This electrode assembly is placed on the scalp (Fig 1A), in our example over the motor region (M1). A return electrode is placed at over the contra-lateral orbit and is not of concern here. CURRENT STEERING: The electrode assembly receives a fixed total current of 1 mA (with -1 mA collected by the return electrode). The current is actively divided across the electrodes within the electrode assembly. Thus, under an “even” current split, 0.25 mA is delivered to each electrode. Under a “partially uneven” current split 0.5, 0.25, 0.25, and 0 mA current is delivered and under a “fully uneven” split 1.0 mA is delivered to one electrode and 0 mA current to the remaining electrodes. COMPUTATIONAL METHODS: WECS was modeled using a previously develop tDCS FEM workflow [6]. A multi-domain geometric mesh was generated of a head using a combination of 3D imaging data and computer aided design electrodes (Simpleware, Exeter, UK). The mesh was imported into a FEM solver (COMSOL, Burlington, MA), where conductivities [5] were assigned to each tissue/material domain. Boundary conditions were applied (cathode ground, inward current density on rivets, insulated on other external surfaces), and the Laplace equation solved for Voltage (and in turn electric field and current density).

3 Results To illustrate the principles of WECS, we considered a simplified electrode assembly with electrodes inside a saline saturated sponge, placed on the scalp (Fig 1A), under two extremes of electrode current distribution conditions (“even” and “fully uneven”) and one intermediary electrode current distribution (“partial uneven”) (Fig 1B). Streamline plots (Fig 1C) of within sponge current flow demonstrate the distribution of current flow in each case from the electrodes to the skin surface.   As expected, we found symmetry when steering current from fully uneven to even current application, but in each case current spreads across the electrode-assembly. At the electrode-assembly interface with the skin, the current density distribution varied only incrementally across conditions (e.g. less than would be expected with even minor changes in electrode assembly or skin properties; [5]) with no significant difference in peak current density (~2

Journal of Medical Devices. Received March 03, 2015; Accepted manuscript posted March 23, 2015. doi:10.1115/1.4030126 Copyright (c) 2015 by ASME

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A/m2; typically predicted around edges). Thus, with this electrode assembly design even if three of four electrodes failed, current steering to the one functional electrode would not significantly increase current density in the skin; hence, not effecting tolerability. Furthermore, we predicted that the electric field at the brain under all three cases was essentially identical (Fig 1E). Therefore, using this electrode-assembly design, current can be steered across electrodes without effecting current distribution in the brain target. We note the goal of WECS in contrast to current steering in DBS, is not to alter current flow at the target (neuromodulation).

Fig 1: FEM analysis of electrode assembly to validate the underlying assumption of within electrode current steering. (A) Represents a montage with electrode assembly. (B) “Even”, “Partially Uneven”, and “Fully Uneven” current injection mode through metal rivets of an electrode assembly keeping total current constant. (C) Illustrates streamline current flow from each metal rivets under all three current injection conditions. (D) Current density observed at the scalp electrode interface. (E) Presents an electric field distribution found in the brain target.

4 Interpretation Within Electrode Current Steering (WECS) is proposed here as a novel method to increase the tolerability of tDCS without altering underling neuromodulation. Thus, using an exemplary design, we illustrated how current flow in the brain can remain unaltered (Fig 1E) even as current is steered between electrodes inside the electrode-assembly. WECS can be generalized to other noninvasive electrical stimulation technique and potentially to invasive techniques where an artificial or natural electrolyte barrier exists between the electrode and the tissue. For invasive techniques, WECS may complement traditional current steering but be used to protect electrode and tissue from injury. Success of this approach depends on the appropriate design of the electrode assembly (Fig 1A) and the algorithm used to steer current between electrodes – topics to be considered in future design efforts. The essential principles in WECS design relate to producing functional equivalency between current arriving at each electrode as far as current entering the brain target. Specifically, regardless of how a total amount of current is distributed between electrodes, brain current flow is unchanged. A further consideration is how current flow at the skin (scalp) is altered. On the one hand, current steering should avoid significant increases in current density at the skin, maintaining as uniform a current density at the skin as practical. On the other hand, when non-ideal conditions at the electrode or skin arise, including increasingly non-uniform current flow or electrode failure, current steering may be used to compensate. For example, if a given electrode fails and a high over-potential at the electrode is detected, current may be steered to other electrode to minimize electrochemical hazard [4] or if one region of the sponge becomes dry during use, current may be diverted to the most distant electrodes. Inherent to the above concept is the ability to detect non- ideal conditions and program appropriate corrective measures. The simplest feedback is the voltage at each current source, which using signal processing and “test signals” (superimposed currents not used for neuromodulation) or a “sentinel electrode” (not used for DC) may be used to calculate single electrode impedance [3]. Additional information can be derived by using test signals to isolate the impedance of the sponge/electrolyte between the electrodes, generating a prediction for current density patterns that can be corrected.

References [1] Nitsche, MA., Liebetanz D., Lang N., Antal A., Tergau F., Paulus W., 2003,

“Safety criteria for transcranial direct current stimulation”, Clin Neurophysiol., 114(11): 2220-2., doi: 10.1016/j.clinph.2009.03.018

[2] Butson, CR., McIntyre, CC., 2008, “Current steering to control the volume of tissue activated during deep brain stimulation”, Brain Stimul., 1(1): 7-15., doi: 10.1016/j.brs.2007.08.004

[3] Khadka, N., Rahman, A., Sarantos, C., Truong, DQ., Bikson, M., 2015, “Methods for Specific Electrode Resistance Measurement during Transcranial Direct Current Stimulation”, Brain Stimul., 8: 150-159

[4] Poreisz C, Boros K, Antal A, Paulus W., 2007., “Safety aspects of transcranial direct current stimulation concerning healthy subjects and patients.”, Brain Res Bull., 72(4e6):208e14.

[5] Kronberg, G., Bikson, M., 2012, “Electrode assembly design for transcranial Direct Current Stimulation: A FEM modeling study”, Conf Proc IEEE Eng Med Biol Soc.

[6] Sadleir RJ, Vannorsdall TD, Schretlen DJ, Gordon B., 2010, “Transcranial direct current stimulation (tDCS) in a realistic head model.”, Neuroimage., 41(4):1310-8

Journal of Medical Devices. Received March 03, 2015; Accepted manuscript posted March 23, 2015. doi:10.1115/1.4030126 Copyright (c) 2015 by ASME

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